1. Field of the Invention
The present invention relates generally to image forming apparatuses, and more particularly to an image forming apparatus including a power storage unit that supplies stored power to a heating part.
2. Description of the Related Art
Image forming apparatuses such as copiers, printers, facsimile machines, and MFPs (MultiFunction Peripherals) have a process of fixing heated toner onto plain paper. Heat roller fixing is often employed as a fixing method because of the need for high speed and safety. According to heat roller fixing, a heating roller heated by a heating member such as a halogen heater and a pressure roller disposed opposite the heating roller are in press contact with each other. Plain paper is caused to pass between the heating roller and the pressure roller, so that an unfixed toner image is fixed on the plain paper by heat and pressure.
Generally, the heating roller employs a roller whose core bar is formed of metal such as iron or aluminum, and has large heat capacity. Therefore, at the time of use, the heating roller requires a long rise time of a few to tens of minutes before rising to a usable temperature of approximately 180° C., thus requiring a user to wait for a long time.
Increasing the power capacity of the heating member such as a halogen heater makes it possible to apply large current, thus enabling reduction in the waiting period of the user. However, this is not desirable because if the commercial power supply is around 100 V, excessive cost is required for an apparatus and interconnections for applying large current.
It is also possible to decrease the heat capacity of the heating roller in order to reduce the rise time of the heat roller. However, decreasing the heat capacity of the heating roller causes a drop in the temperature of the heating roller in the case of successively forming images, thus producing a waiting period every time a predetermined number of copies are printed.
Therefore, in the image forming apparatus, power is supplied to the heating roller even in its standby period in which a user does not perform image forming, thereby maintaining the temperature of the heating roller at a preheating temperature slightly lower than the usable temperature of the heating roller so that the heating roller rises to the usable temperature in a short time at the time of use. Accordingly, extra energy unnecessary for image forming is consumed in the standby period in which the image forming apparatus is not in use.
However, a recent increase in environmental conservation activities has required the image forming apparatus to have more advanced energy-saving features. It is considered that reduction in standby energy consumption, which occupies a large proportion of the entire power consumption of the image forming apparatus, has a significant power saving effect.
Therefore, there is proposed a technique to reduce the startup time of a fixing unit by providing an auxiliary power supply (a secondary battery), charging the auxiliary power supply during the standby period of the fixing unit, and supplying power to the fixing unit from a main power supply unit and the auxiliary power supply when the fixing unit is started (see, for example, Patent Document 1).
However, the technique disclosed in Patent Document 1 generally employs a lead storage battery, a nickel-cadmium battery, or a nickel-hydrogen battery as the secondary battery. Therefore, the secondary battery is characterized in that the capacity is degraded and reduced by repeated charging and discharging and that the useful service life becomes shorter as a discharge current becomes larger. Further, there is also the phenomenon of capacity reduction due to the memory effect. Accordingly, this technique is not sufficient to be used as the image forming apparatus.
In view of this, there is proposed a technique to control electric energy supplied to an auxiliary heater by using an auxiliary power supply, employing a chargeable and dischargeable capacitor as the auxiliary power supply (see, for example, Patent Document 2). According to this technique, a heating roller is heated with not only power supplied from a commercial power supply but also power supplied from the auxiliary power supply. The supplied electric energy is controlled by starting or stopping supplying power from the capacitor in accordance with the temperature of the heating roller.
[Patent Document 1] Japanese Laid-Open Patent Application No. 10-282821
[Patent Document 2] Japanese Laid-Open Patent Application No. 2003-297526
However, an image forming apparatus using the capacitor disclosed in Patent Document 2 requires a dedicated charger separately from the capacitor, thus being higher in cost and larger in installation space.